Evolution of structural defects associated with electrical degradation in AlGaN/GaN high electron mobility transistors

Abstract

We have investigated the surface morphology of electrically stressed AlGaN/GaN high electron mobility transistors using atomic force microscopy and scanning electron microscopy after removing the gate metallization by chemical etching. Changes in surface morphology were correlated with degradation in electrical characteristics. Linear grooves formed along the gate edges in the GaN cap layer for all electrically stressed devices. Beyond a critical voltage that corresponds to a sharp increase in the gate leakage current, pits formed on the surface at the gate edges. The density and size of the pits increase with stress voltage and time and correlate with degradation in the drain current and current collapse. We believe that high mechanical stress in the AlGaN layer due to high-voltage stressing is relieved by the formation of these defects which act as paths for gate leakage current and result in electron trapping and degradation in the transport properties of the channel underneath.

Electrical and structural degradation of GaN high electron mobility transistors under high-power and high-temperature Direct Current stress

“…Following the electrical stress experiment, a three-step process 9,19 was followed to remove the passivation and metal contacts from the surface of the devices. Our chemical techniques are rather benign on GaN.…”

“…We find similar pit and trench formation under the gate edge on the drain side as observed by other authors after stress in the OFF-state regime. 19,20 In a new finding, this damage strongly correlates with the degree of drain current degradation suffered by the device. Channel temperature was found to be an accelerating factor of this type of structural degradation.…”

“…26 It has also been found that the damage is accelerated with stress voltage and temperature and correlates with the drain current degradation suffered by the device. 19,20,[26][27][28] In power amplifier applications, the device is typically biased in the ON state. In spite of its importance, there has been very little research on the impact of prolonged highpower stress on the structural degradation of the device.…”

“…[1][2][3][4][5][6][7][8][9] Several mechanisms have been postulated to explain the various degradation patterns that have been observed. [8][9][10][11][12][13][14][15][16][17][18] Several studies [19][20][21][22][23][24][25] have shown the appearance of prominent physical damage (dimples, grooves, pits, trenches, and cracks) on the semiconductor surface under the edge of the gate after prolonged OFF-state stress. The damage extends through the GaN cap and the AlGaN barrier layer and in some extreme cases reaches into the GaN buffer layer.…”

On the link between electroluminescence, gate current leakage, and surface defects in AlGaN/GaN high electron mobility transistors upon off-state stress

“…In order to increase the lifetime of AlGaN/GaN HEMTs, it is essential to have a detailed understanding of the mechanisms underlying the degradation of these devices. It has previously been reported that the gate leakage current, I g , degrades when the devices are operated in pinched off conditions or when the gate Schottky contact is reverse biased 2,3,4,5,6,7,8,9,10,11,12 . Some reports have correlated the increase in I g during offstate stress of the devices with the appearance of grooves and v-shaped pits on the surface of the semiconductor, located at the edges of the gate contact 6,7,9,12 .…”